The increased usage of electronic equipment such as computers and other digital devices has lead to a concern for the hazards associated with electromagnetic radiation such as radar waves, microwaves, and electromagnetic radiation produced by electronic circuits. Because of these concerns, electromagnetic shielded composites have been developed to: 1) protect the user of an electronic device, 2) protect an electronic device, and 3) protect surrounding electronic devices. As the electronic industry grows there in an increasing need for improved electromagnetic shielded materials which may be incorporated into electronic products.
Plastic articles formed with electrically conductive materials are particularly convenient as compared to traditional metal materials because they are light weight, easily produced using injection molding techniques, and of low cost. Typically these electrically conductive materials are composites of plastics and conductive fibers.
Various conventional techniques have been employed when incorporating electrically conductive fibers into a polymer matrix to make electromagnetic shielded composites. A drawback to these techniques is their inability to provide for adequate dispersions of conductive fibers within the composites. A technique which yields poor dispersions of conductive fibers in a composite requires the use of larger amounts of fibers in order to obtain effective electromagnetic shielding. To solve this problem, conventional techniques have employed several mechanical means to intimately mix conductive fibers a polymer to make a composite product. Unfortunately, the mechanical mixing of conductive fibers with a polymer is stressful and causes damage to the fibers such as fracture or breakage. These damaged fibers impart reduced electromagnetic shielding properties due to their reduced ability to conduct electricity through the composite article.
An example of a basic technique for making an electromagnetic shielded composite involves heating a thermoplastic to a molten temperature and then kneading in the conductive powders fibers. Unfortunately, when kneading conductive fibers with a molten thermoplastic, the fibers are often broken due to the cutting action by the kneading screw and by the shearing of the resin. These fibers are broken into smaller and smaller segments such that the resulting composite article contains only shorter length broken fibers. Such shortened fibers impart reduced electromagnetic shielding properties to the composite article due to their reduced ability to conduct electricity through the composite article. Composite articles formed with broken fibers require the use of higher amounts fiber and may lead to embrittlement of the composite article thus formed. Additionally, operators working directly with the cut fibers and powders can experience pain or itchiness in handling the materials.
To avoid the problems with directly mixing in cut fibers, attempts have been made to provide electromagnetic shielding plastic compound pellets by impregnating conductive fibers with a polymer and then cutting the impregnated fiber into pellet form. An example of such a process involves the use of continuous lengths of filaments which are passed through a bath containing a molten resin whereby such filaments become impregnated with the polymer. Once the filaments are impregnated they are continuously withdrawn from the bath, commingled either before or after passage through a heat source and cooled to solidify the molten resin around the fibers. These impregnated fibers are then cut to form pellets which are then formed into composite articles. Another example of an impregnation technique involves the use of a conductive tow comprised of a plurality of strands. The tow is mechanically splayed allowing for the impregnation of a polymer between the strands and then the strands subsequently gathered together into an impregnated tow which is cooled and chopped into pellets. There are various disadvantages to these impregnation techniques. One disadvantage is that impregnation techniques are relatively slow and cumbersome. Additionally, impregnation techniques often do not provide adequate integration of polymer and fiber. Impregnated fibers often fray when cut into pellets and can become separated from the resin. When consolidated into a composite, pellets made by impregnation techniques often provide an inadequate dispersion of fibers and poor electromagnetic shielded ability. It is believed this is due, at least in part, to the inadequate integration of polymer and fiber resulting from impregnation techniques.
A possible solution to the problems associated with impregnation techniques is to encase or coat fibers with a polymer sheath. For example, U.S. Pat. No. 4,530,779 to Mayama et al., discloses initially coating a strand of fibers with a coupling agent and subsequently coating the strand with a polymer. The coated strand is then chopped into pellets. Other attempts at forming electromagnetic shielded articles have passed electrically conductive fiber strands through a bath of a polymeric material to first impregnate the fibers. These impregnated strands are then encased with a second polymeric material as exemplified by U.S. Pat. No. 4,664,971 and U.S. Pat. No. 5,397,608 both to Soens. The encased strands are then chopped into pellets. A disadvantage of the aforementioned methods exemplified by Mayama et al. and Soens is that these methods produce pellets, which by themselves, are not adequate to form an electromagnetic shielded composite. Additional polymer material must be added to the pellets resulting in an additional mixing step which often causes mechanical damage such as fracture or breakage of conductive fibers. Mechanical damage to the conductive fibers results in a composite with poor electromagnetic shielding. Another method, as described in U.S. Pat. No. 4,960,642 to Kosunga et al., discloses impregnating conductive fibers with an oligomer and encasing the resulting impregnated bundled fibers in a polymer. The encased bundled fibers are then chopped into pellets. A major drawback to the method of Kosunga et al. is that the fibers must be impregnated under pressure.
Accordingly, there is a long felt need in the art for a method which provides an adequate dispersion of electrically conductive fibers in a polymer matrix to make an electrically shielded composite. The present invention provides for such a method without any of the disadvantages identified in conventional methods as exemplified above. Unlike conventional methods, the present invention provides for a composite with improved electrical shielding properties and avoids undesirable mechanical damage to conductive fibers. The present invention also provides for pellets, which by themselves, may be consolidated into an electrically shielded composite and avoids the undesirable step of mixing additional polymer material to the pellets. Furthermore, the present invention avoids the complex and/or cumbersome impregnation techniques found in conventional methods.